Conventional Schottky-type photoconductive elements comprise, as illustrated in FIG. 1 (not to scale), a glass substrate 1, a metallic electrode 2, an amorphous silicon film 3, and a transparent electrode 4. When used in air, however, they suffer from a disadvantage in that the durability (service life) is deteriorated. Thus, to protect the sensor surface from dust, for example, in air, or against humidity, a film of SiO.sub.x or Si.sub.3 N.sub.4 has conventionally been provided by a plasma CVD (chemical vapor deposition) method as a passivative film (a transparent, insulative protective film). When, however, such a film is provided by a plasma CVD method, the transparent electrode on the sensor surface is often damaged. For this reason, it has been desired to develop a film which can be used in place of the SiO.sub.x or Si.sub.3 N.sub.4 film.